Many analytical and filtration applications that involve gaseous or liquid fluids require the use of chemically inert media which are capable of removing particles in the micron and submicron ranges. Microporous media employed for such purposes are typically relatively delicate structures which are not capable of holding a fixed shape and which are also easily damaged. For example, U.S. Pat. No. 4,431,545 describes a hydrophilic, microporous filter system having ultrafiltration capability, i.e., the ability to remove particles as fine as about 0.001 to about 10.mu.. A preferred filter medium for use in the system described in U.S. Pat. No. 4,431,545 is a thin (typically, only a few mils thick) microporous polyamide membrane. This membrane is difficult to work with because of its limited strength and its lack of internal rigidity.
Some of the most chemically inert materials available for use as microporous membranes include those polymeric materials known as Teflon.RTM.. This term represents fluorocarbon resins such as polyperfluorinated olefins formed from monomers containing 2 and 3 carbon atoms, such as polymers formed from tetrafluoroethylene (TFE), fluorinated ethylene-propylene (FEP) and copolymers thereof. It is the chemical inertness of these materials which makes them desirable for analytical and many filtration applications. In addition, polytetrafluoroethylene (PTFE) membranes provide the desirable properties, when used for analytical purposes, of generally having a white background and being translucent when wet. In the forms suitable for use as filtration or analytical media, microporous membranes of PTFE are thin, relatively delicate structures with little internal rigidity, having very low flexural moduli. That is, they do not retain their shape when unsupported, typically hanging limply in loose folds, much as a piece of thin cloth drapes when unsupported. These drawbacks create difficulties in working with such membranes. Thus, corrugating such media to increase the surface area available in a filter element or even using as simple planar filters subjects such media to easily being torn or, because of static electrical charges, to adhering to itself. The lack of commercial availability of PTFE membranes in the form of flat disks of thin, fine pored media, a highly desirable filtration material for many applications, provides an additional indication of the difficulty of handling and working with such delicate materials. Besides having a low flexural modulus and limited strength, PTFE and similar membrane materials tend to become electrostatically charged. Such a material sticks to itself and makes it difficult to maintain in a flat or planar disk form. Furthermore, because of the low flexural modulus and the difficulty in sealing PTFE membranes to support materials, such as polypropylene housings, it is difficult to manufacture filter structures which incorporate this type of membrane in such housing. For example, in biomedical applications particularly, it is frequently desirable to insert a precut flat or planar filter piece into a preformed support structure and to tightly seal the periphery of the filter piece to the support structure. Unsupported PTFE, because of its low flexural modulus, cannot be readily used in such a manner, since accurate placement becomes difficult as does sealing the membrane to the support structure.
Currently, available from Pallflex Products Corporation is a supported PTFE membrane. The PTFE membrane is mounted on an annular polyolefin or polyester frame member which serves to prevent significant flexing of the membrane. The outer and inner diameters of the frame member are 47 and 37 mm, respectively. While such an arrangement is effective for PTFE membranes of about this size, it is not particularly effective for larger PTFE membranes or for non-circular shapes.
Finding suitable materials and structures to support the flimsy films of Teflon.RTM. has presented some difficulties. Thus, because of its chemical inertness, Teflon.RTM. does not easily adhere to other materials even when heat-bonding is attempted. In addition, in certain analytical procedures, including analyses where translucent Teflon.RTM. is critical, the presence of any sort of non-Teflon.RTM. material, such as a material used as an adhesive, potentially introduces a foreign contaminant, due either to leaching or to degradation. Such contamination frequently adversely affects those media used in analysis. This tends to be particularly true when materials such as polypropylene are used.
To provide suitable strength and greater rigidity, an approach used with other polymeric membranes having similar structural and physical properties has been to incorporate a substrate, such as a fibrous web or mat, permanently into the membrane structure. This both enhances the strength and the flexural modulus of the membrane. While such internally supported membranes are more easily handled and resist tearing, which would be disastrous for a filtration material in most applications, the shortcoming of such media is their greater resistance to fluid flow. That is, such materials frequently show a several fold increase in pressure drop across a membrane compared to unsupported membranes. In addition, some of the materials used for the supporting web may contribute to a reduced chemical compatibility; that is, the medium may have insufficient chemical or physical resistance to chemical reagents, solvents and the like.
Currently, many filtration procedures are also limited by the physical properties, other than those discussed above, of the filtration media employed. For example, many of the currently used filtration media cannot be used in high temperature applications because the materials show a tendency to either pyrolyze, otherwise decompose, sinter or fuse. Were a medium to exist which overcame the aforementioned problems and could also be used in applications which require higher temperatures, the additional versatility would result in much greater commercial utility of such a medium.